The present invention relates to a process for the preparation of microcapsules having a capsule wall of a polymeric substance enclosing a water-immiscible material, by interfacial polymerization of an aqueous dispersion of a first polymer-forming substance dissolved in a water-immiscible material with a second polymer-forming substance which reacts with the first one under the mixing conditions, to form a stable aqueous suspension of microcapsules of the water-immiscible material. The invention also relates to a composition comprising said microcapsules and the use of said microcapsules.
It is known per se to prepare microcapsules by carrying out an interfacial polycondensation reaction in a liquid, with one of the reactive components required to form the capsule wall being dissolved in the disperse phase and the other being dissolved in the continuous phase. Such a process is disclosed for example in U.S. Pat. No. 3,577,515. This process is carried out by initially dispersing a solution of the first reactive component required to form the capsule wall and an immiscible material in a continuous phase with high shear and then adding a solution of a second complementary reactive component dissolved in the medium containing the continuous phase to it with mixing. In the process of U.S. Pat. No. 3,577,515, it is recommended to use nonionic protective colloids such as polyvinyl alcohol, gelatin and methyl cellulose for the dispersion of a water-immiscible organic phase in a continuous aqueous phase. A variety of wall-forming components are disclosed.
One drawback of this process is that it is incapable of encapsulating concentrated amounts of material, such as 45% by weight or more, because the microcapsules tend to agglomerate into large unusable masses.
Several U.S. patents such as U.S. Pat. Nos. 4,280,833, 4,640,709 and 4,938,797 disclose processes for microencapsulation via an interfacial polycondensation reaction whereby concentrated amounts of water-immiscible materials, such as 45% by weight or more, can be encapsulated within a polymeric shell wall, with the finished microcapsules forming a stable suspension in an aqueous medium. The ability to obtain high concentrations of microcapsules is taught to critically depend on the use of a specific type of surfactant or a combination of surfactants, for example, a salt of a lignin sulfonate in U.S. Pat. No. 4,280,833, an alkylated polyvinyl pyrrolidone polymer in U.S. Pat. No. 4,640,709 and a combination of a formaldehyde condensate of a naphthalene sulfonate with ethylene oxide/propylene oxide/ethylene oxide block polymers in U.S. Pat. No. 4,938,797.
Microcapsules prepared by interfacial polycondensation can advantageously contain materials which have a variety of uses, such as for dyes, inks, color formers, pharmaceuticals, cosmetics, flavoring materials, agricultural chemicals and the like. Any liquid, oil, low melting solid or solvent-soluble material into which a first wall-forming material can be dissolved and which is non-reactive with said wall-forming material may be encapsulated with this technique. Once encapsulated, the liquid or other form is preserved until it is released by some means which breaks, crushes, melts, dissolves, or otherwise removes the capsule skin or until release by diffusion is effected under suitable conditions.
The process of the present invention is particularly suitable for the production of microcapsules of very small capsule size which are suspended in an aqueous medium and which contain agricultural chemicals such as pesticides and herbicides.
Aqueous suspensions of pesticide and herbicide microcapsules are particularly useful in controlled release pesticide and herbicidal formulations because they can be diluted with water or liquid fertilizer and sprayed using conventional equipment, thereby producing uniform field coverage of the agricultural chemical. Additives such as film forming agents can be added directly to the finished formulation to improve the adhesion of microcapsules to foliage. In some instances, reduced toxicity and extended activity of encapsulated herbicides and pesticides have been noted.
Most conventional oil-in-water surfactants fail to produce emulsions which are suitable for microencapsulation via interfacial polymerization. More importantly, they fail to maintain a stable suspension of microcapsules during the critical wall-forming step, particularly when high concentrations of water-immiscible materials are involved. This results in coalescence of the partially formed capsules into larger agglomerates which cannot be redispersed, or even solidification of the entire nonaqueous phase when the second reactive component is stirred into the oil-in-water emulsion. The critical factor in achieving stable, high concentrations of microencapsulated materials is therefore the selection and use of a special type of surfactant or adjuvant which is able to both produce a stable oil-in-water emulsion and, more importantly, to stabilize the subsequent suspension of capsules during the wall-formation step so that coalescence to large unusable agglomerates is avoided.
It has now been discovered that by using salts of partial esters of styrene-maleic anhydride copolymers in the process of microencapsulation via interfacial polymerization, it is possible to obtain highly concentrated, stable microcapsule suspensions. The the present invention thus provides a new and improved method of encapsulation via interfacial polymerization which provides a rapid and effective way to obtain high concentrations of water-immiscible materials in the form of an aqueous microcapsule suspension.
Generally, the water-immiscible material can be effectively encapsulated at concentrations up to about 45% by weight or higher without difficulty. Once the water-immiscible material, for example, a pesticide or herbicide, is encapsulated, the system becomes a solid-in-liquid suspension which can be directly combined with other water-based materials, for example, liquid fertilizers. Or it can be diluted with water and sprayed by a conventional applicator.